This invention relates generally to rotating filter discs, and, more particularly, to filter disc sectors which have controlled internal volumes in order to limit the volume of filtrate in transit through the sectors at any time.
Many disc filters are disclosed in the prior art to filter slurries in the paper and pulp industries. In larger sizes, the discs are, typically, made-up of adjacent sectors mounted on a generally hollow rotating shaft so that successive sectors of the disc are rotated into, and out of, the slurry. During the immersion of each sector in the slurry, a pressure differential is applied causing the liquid in the slurry to flow along flow channels on each sector axial surface, into a drainage bore, and thence into a core drainage channel in the hollow shaft, while a filter cake builds up on the filter bag or face wire on the outside surface of each sector. Upon emergence of a sector from the slurry, the filter cake is removed, first by ceasing application of the pressure differential, and then, by application of a knock-off shower designed to peel away the substantially dry filter cake. This cake is then collected while the sector re-enters the slurry to begin another cycle.
Demands for increased filtration capacities have led to increasing filter sizes, with some commercial applications now ranging to 18' or more in diameter. For slow draining low consistency pulps (Approx. 0.5 to 2.0% solids, by weight) a large volume of liquor must be removed to significantly thicken the slurry. This liquor must be reprocessed or otherwise disposed of. Since, as the filter cake develops, it acts as a fine filter medium, it is expected that the first liquor collected will be less clean (or contain more fine fibers) than will that which is separated later in the cake building cycle. Thus, it would be economically beneficial to split the filtrate into fractions corresponding to its fiber content in order to reduce handling and unnecessary reprocessing costs. Ideally, all fiber-bearing filtrate would return to the feed slurry, while all fiber-free filtrate would be disposed of or re-used, as appropriate. In order to make such a split, it is necessary to drain the filtrate very rapidly in order to not mix the early (cloudy) filtrate with the later (clear) filtrate.
Disc filters preferably operate within a carefully defined set of parameters which at times may appear to conflict. That is, for maximum filtration capability, a large surface area is dictated; at the same time, to maximize rapidity of drainage or filtrate discharge from the disc filter, the so-called "transit volume"--i.e. that volume which collects, contains, and subsequently discharges, the filtrate--must be small. The transit volume of the disc filter, is defined as the leaf volume plus the core volume, i.e. the total internal volume of all the sectors of a disc filter plus the volume of the core drainage channel, usually within the hollow rotary shaft.
For effective separation, it is best to maximize the scavenging ratio (SR) which is defined as the filtrate flow rate (gpm) divided by the product of the transit volume (gals. capacity) and the rotary frequency (rpm) of the disc. A scavenging ratio of four or higher is desirable for adequate filtrate split.
From this it is seen that SR is increased by increasing filtrate flow rate (pumping rate) or by decreasing transit volume or disc speed or both. Since decreasing disc speed reduces capacity, transit volume is the preferred control variable, given an established pumping rate or filtrate flow rate.
Thinner discs have been tried for assuring rapid discharge by minimizing transit volume, but they may result in discs, or sectors, which do not have adequate strength to resist the large drag forces and crushing forces experienced by the sector during rotation into and out of the slurry. Drag forces are due to the apparent viscosity of the slurry, while crushing forces are due to the pressure required to drive the filtrate through the filter medium. Thus, the sector thickness must be great enough to provide adequate mechanical strength and durability to withstand the large forces attendant on the large surface area of the sectors. Sectors are commonly fabricated from stainless steel in order to achieve maximum strength and durability while maintaining minimum sector weight and cost. The leaf volume, because of the sector thickness required to withstand forces described above, often is still too large to achieve the desired scavenging ratio of four or more.
Attempts to strengthen the sectors by using heavy gauge materials increase the weight to the extent that lifting equipment is required for handling the sectors during face wire or screen replacement, and reinforcement of the filter core (or hollow shaft) is required. The leaf volume usually remains too large to achieve the desired high scavenging ratio. To compensate for the leaf volume, attempts have been made to seal the hollow volume beneath the leaf (or sector) deck, i e., the imperforate sector surface having flow channels open to the filter surface for draining filtrate to the core drainage channel When these seals fail and leak, the hollow volumes fill with liquor, and drastically increase sector weight and distortion loads on the filter core, thereby increasing wear and tear on the filter
From this it is clear that the parameters which determine optimal filtering performance are not always compatible with economy, strength, reliability, and ease of maintenance.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.